Spacecraft in low earth orbit (LEO) experience a variety of hazards including exposure to micrometeoroids and orbital debris (MMOD). Average impact speeds for orbital debris on spacecraft in LEO are 9 to 10 km/s, and 20 km/s for micrometeoroids. Due to their high speeds, MMOD can cause considerable impact damage to sensitive spacecraft surfaces such as windows, structural elements, electronic boxes, solar arrays, radiators, thermal protection system (TPS) materials covering crew/cargo return vehicles, as well as crew modules. Prolonged exposure to the on-orbit MMOD environment can potentially compromise the TPS covering return vehicles such as the future crewed vehicles expected to visit and remain for a half-year or longer at the International Space Station (ISS).
However, determination of MMOD impact on orbiting spacecraft currently requires visual inspection. For human-rated spacecraft (such as the ISS and, previously, the Space Shuttle Orbiter), this has required crew time as well as vehicle assets to identify damage due to MMOD strikes. For unmanned spacecraft, there are no human assets present to conduct detailed surveys to ascertain potential damage.
Orbital debris fragments are generated by on-orbit explosions, collisions, breakups and degradation and is a growing threat as suggested by recent orbital debris models. Two recent events, the Chinese ASAT test in 2007 and Cosmos/Iridium collision in 2009 significantly increased orbital debris for satellites in sun-synchronous orbit. The MMOD threat is real and poses considerable risk to crewed, and un-crewed, orbiting spacecraft.
While the current practice of visual inspection may successfully indicate the location of a debris strike, it does not currently allow precise determination of exactly when the debris strike occurred. It is only possible to determine that a debris strike has occurred between two successive inspection events unless damage to other components can allow inference that a debris strike occurred at a specified time.
The Structural Health Monitoring (SHM) community has experimented with using digital signal processing of acoustic signals with Acoustic Emission/PZT/Piezoelectric sensors to automatically detect MMOD impacts. While such approaches have had some success qualitatively detecting that an impact occurred, current systems cannot quantify the location of a MMOD impact and/or the severity of the MMOD impact.